PROJECT SUMMARY Ricin is a category B toxin due to its ease of acquisition, dissemination, and the high potential for morbidity and mortality after exposure. Ricin is naturally produced by the castor bean plant, Ricinus communis, which is cultivated on industrial levels worldwide. Ricin constitutes up to 5% of the total dry weight of the castor bean and can be extracted using several simple enrichment steps. Ricin in semi-purified or purified form is extremely toxic to humans following injection, inhalation, or ingestion and has been used as an agent of bioterrorism. Ricin was previously weaponized by the United States, the former Soviet Union, Iraq and likely other countries, has been used in assassinations and was recently detected in a number of U.S. government facilities. There are currently no methods of preventing or treating ricin exposure ? this represents a major unmet need for protection of civilians and military forces. The Mapp team, in collaboration with Dr. Nicholas Mantis (Wadsworth Center, Albany, NY) and Dr. Chad Roy (Tulane National Primate Research Center, Covington, LA), has identified a number of highly potent anti-ricin monoclonal antibodies (mAbs). These mAbs, either in their original fully murine state, when chimerized with human constant regions, or when fully humanized, have demonstrated protection of mice when administered prior to and after ricin challenge. Further, one of these mAbs, humanized PB10 (hPB10), has been shown to protect non-human primates (NHPs) from aerosol challenge therapeutically ? this is the first demonstration of protection of NHPs by a therapeutic against a lethal aerosol challenge. We have now generated evidence that a mAb cocktail consisting of one RTA and one RTB mAb may provide improved potency and extend the therapeutic window. The in vitro and in vivo potency of the mAb cocktails we have tested is superior to any individual mAb previously described, which is consistent with the combination of mAbs functioning at different steps in the intoxication process. Anti-RTB mAbs interfere with ricin attachment to target cells, while anti-RTA mAbs are proposed to block ricin intracellular trafficking. The Specific Aims to advance a ricin medical countermeasure to the clinic are: 1) Engineer, produce, and down-select from the panel of RTA/RTB mAbs; 2) Humanize the lead RTA/RTB mAb cocktail and manufacture to support Aim #3; 3) Determine the prophylactic and therapeutic efficacy of the lead RTA/RTB combination in the rhesus macaque aerosol challenge model. These data combined with the Chemistry, Manufacturing and Control (CMC) information will form the basis of a pre- Investigational New Drug (IND) meeting with the FDA to solicit guidance on continued development of the product.